In the following, first of all the basics regarding the operation of cochlear implants are discussed to improve and/or facilitate an understanding of the present invention. Patients with a complete loss of their hearing sense often receive so-called cochlear implants which enable the generation of sound stimuli by electric impulses in the inner ear. For a better understanding, FIG. 7a shows a schematical illustration of a human ear, wherein the cochlea is illustrated in a rolled-off form. As it may be seen from the schematical illustration 700 of FIG. 7a, a sound stimulus generates an oscillation on a basilar membrane of the cochlea which is converted into neural impulses with the help of hairs and associated auditory cells. In a healthy hearing system, a conversion of audio frequencies into positions along the basilar membrane takes place in the cochlea. In other words, depending on the frequency of an incoming sound, an area of the basilar membrane corresponding to the frequency is excited especially strongly.
FIG. 7b shows a schematical illustration of a human ear into which electrodes of a cochlear implant are introduced. From the graphical illustration 750 of FIG. 7b it may be seen that a cochlear implant comprises several electrodes 760 which are placed at different positions along the basilar membrane in the cochlea. By suitable impulse patterns applied to the electrodes 760, a cochlear implant may thus perform an approximation, even if only coarse, of the stimuli which occur in a healthy hearing system. It is to be noted here that a cochlear implant typically directly excites one auditory nerve and/or several auditory nerves.
A conventional cochlear implant here comprises a microphone for sound reception and a signal processing unit for converting the sound signal into suitable stimulation impulses. The quality of the sounds heard by a patient with a cochlear implant is here limited by several factors. On the one hand, the number of implantable electrodes is limited, so that a frequency resolution of a healthy ear cannot be achieved. Further, the number of impulses which may be generated per time unit is strongly limited by a transmission path to the electrodes. Finally, the efficiency of the signal processing unit is further usually limited by a limitation of the current reception for achieving a sufficient operation time with a predetermined battery capacity.
In the following, a conventional signal processing for the cochlear implant is described. Currently, conventional cochlear implants have up to 22 electrodes which may respectively be activated by electrical impulses (stimulation). The respective impulse strength of an electric impulse determines the strength of the stimulus which is passed on via the auditory nerve to the brain. The position of an electrode here corresponds to the pitch or frequency of a tone, respectively, which would cause a maximum alignment of the basilar membrane of the cochlea in a healthy hearing system at the corresponding location, i.e. at the position of the electrode. Thus, conventionally, in the signal processing unit the incoming sound signal is split up by bandpass filters into the corresponding frequency portions, wherein for this purpose, for example, a filter bank is needed. The needed impulse strength for electrodes at different positions may then be derived from a signal strength in the associated frequency bands.
As conventionally at a time only one electrode may be stimulated, the control takes place sequentially, for example in a multiplex. Here, the sequence of the controlled electrodes may vary. If all electrodes were controlled in succession, due to the limitation of the overall stimulation rate for each individual electrode a time resolution would result which is too low. Thus, a method was developed which, in a certain time section, selects a lower number of electrodes according to the corresponding signal strength. If the stimulation in a cycle is limited to N selected electrodes with an overall number of M electrodes, this is a so-called NofM strategy, which is also referred to as an advanced combinational encoder (ACE). A detailed description of the advanced combinational encoder may, for example, be found in the technical reference manual “ACE Speech Coding Strategy” (Nucleus Technical Reference Manual, Z43470 Issue 3, Cochlear Corporation, December 2002). The overall stimulation cycle is thus shortened from M impulses to N impulses, which leads to an increase of time resolution. Conventional numerical values here are M=22 and N=8.
The NofM strategy comprises the problem that the frequency ranges with the highest signal strengths are not the most important ones for perception. Thus, by the Laboratory for Information Technology of the University of Hanover (Germany) in cooperation with the Auditory Center Hanover (Germany) approaches for improvement were developed which use psychoacoustic models for selecting the electrodes, as they are also used in audio encoding. For this new method, also referred to as psychoacoustic advanced combinational encoder (PACE) the computing power of existing signal processing units is still sufficient. Examinations with patients have further indicated that the mentioned approach may increase speech intelligibility.
FIG. 8 shows a schematical illustration of conventional devices for coupling audio signals into a cochlear implant. The schematical illustration of FIG. 8 is designated by 800 in its entirety. The schematical illustration 800 describes a transmission path of audio signals from an audio signal origin to a cochlear implant. With today's conventional digital transmission of audio signals it is normal to encode an audio signal 810 in an encoder 820. The encoder may, for example, be part of a (broadcast) transmitter and be realized in hardware or in software. An encoded audio signal 822 is transmitted via a broadcasting link in a radio transmission and then received by a receiver 830. In the receiver, typically a frequency conversion and further a decoding takes place. The receiver may, for example, be a broadcasting receiver or a television receiver. For radio or television reception an acoustic transmission from the broadcast (radio or television) receiver 830 to a signal processing unit 850 of the cochlear implant 834 is a simple and advantageous possibility. In this case, the audio signal is reproduced by the broadcast receiver 830 via a loudspeaker 840. The cochlear implant includes in the mentioned case a microphone 844 which receives the decoded audio signal reproduced by the loudspeaker 840 and passes the same on to the signal processing unit 850 located in the cochlear implant. The signal processing unit 850 thereupon generates stimulation impulses for the excitation of auditory nerves based on the audio signal provided by the microphone 844. The stimulation impulses may then be supplied to electrodes which excite the auditory nerves of a human patient.
It is further sometimes advantageous to directly transmit a decoded audio signal provided by the decoding to the cochlear implant 834. For such a direct transmission, for example induction loops 860, 862 may be used, whereby the transmission is insensitive towards interfering noise. If instead of a broadcast receiver a mobile player is used, like, for example, a portable CD player or an MP3 player, then usually the audio signal and/or sound signal of the signal processing unit 850 of the cochlear implant 834 may also be directly supplied via a cable.
In summary it may be noted, that the cochlear implant 834 conventionally receives an audio signal which was decoded by a broadcast receiver or a media player. It has been shown, however, that the conventional concept does not guarantee an optimal speech quality.